Fuel injection control apparatus and fuel injection control method for internal combustion engine

ABSTRACT

An internal combustion engine includes an in-cylinder injection valve and an intake passage injection valve. A control apparatus of the engine determines whether the temperature of a component located in the exhaust passage is higher than or equal to a predetermined temperature. When the temperature of the component is higher than or equal to the predetermined temperature, the control apparatus increases, without changing the total amount of fuel supplied to the cylinder, the ratio of a fuel injection amount of the intake passage injection valve to a fuel injection amount of the in-cylinder injection valve compared to a case where the temperature of the component is not higher than or equal to the predetermined temperature. Therefore, the catalyst is reliably prevented from being overheated, while preventing the fuel economy from deteriorating.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus and a method forcontrolling fuel injection in an internal combustion engine thatincludes an injection valve for injecting fuel into a cylinder and aninjection valve for injecting fuel into an intake passage. Particularly,the present invention relates to a technique for preventing a componentprovided in an exhaust passage, such as catalyst for purifying exhaustgas, from being overheated.

Japanese Laid-Open Patent Publications No. 5-231221, No 7-103050, andNo. 2001-20837 each disclose an internal combustion engine having aninjection valve for directly injecting fuel into a cylinder (in-cylinderinjection valve) and an injection valve for injecting fuel into anintake passage (intake cylinder injection valve). In such an internalcombustion engine, various fuel injection modes can be performed usingthe two types of injection valves, thereby permitting the engine to befinely controlled according the engine operational state. For example,when the intake passage injection valve is used for injecting fuel,homogeneous air-fuel mixture is formed in the cylinder and such mixtureis combusted (homogeneous combustion).

On the other hand, when the in-cylinder injection valve is used forinjecting fuel, the engine is operated in a combustion mode selectedfrom a stratified combustion mode and a homogeneous combustion mode. Inthe stratified combustion mode, fuel is injected from the in-cylinderinjection valve during the compression stroke of the engine so thatrelatively rich air-fuel mixture is formed in the vicinity of theignition plug. In this state, the mixture is ignited to performcombustion. The stratified combustion mode permits the engine to operateby the combustion a relatively lean air-fuel mixture. Accordingly, thefuel economy is improved and the emission of CO₂ is reduced.

In the homogenous combustion mode, fuel is injected from the in-cylinderinjection valve during the intake stroke of the engine so thathomogenous air-fuel mixture is formed in the cylinder. In this state,the mixture is ignited to perform combustion. Air that is drawn into thecylinder is cooled by the effect of heat of evaporation of injectedfuel. Accordingly, the filling efficiency of air to the cylinder isincreased. Therefore, when the engine is operated in the homogeneouscombustion mode, the engine produces greater power.

Also, a fuel injection mode in which fuel is injected from both of thein-cylinder injection valve and the intake passage injection valve canbe performed.

A catalyst for purifying exhaust gas is provided in the exhaust systemof an engine. If the catalyst is overheated due to a temperatureincrease of exhaust gas, the purifying performance of the catalyst canbe degraded, and the life of the catalyst can be shortened. JapaneseLaid-Open Patent Publication No. 2002-130011 discloses a technique inwhich the amount of fuel supplied to a cylinder is increased to preventa catalyst from being overheated. However, the technique disclosed inthe publication is only applicable to engines having an in-cylinderinjection valve, but cannot be favorably applied to engines having anin-cylinder injection valve and an intake passage injection valve.

Japanese Laid-Open Patent Publication No. 7-103050 discloses a techniquefor switching injection valves. Specifically, when an abnormality infuel injection from an in-cylinder injection valve is detected, fuelinjection from the in-cylinder injection valve is stopped and fuelinjection from an intake passage injection valve is started. Anabnormality of fuel injection degrades the combustion state and affectsthe temperature of exhaust gas, in other words, affects the temperatureof a catalyst. However, Japanese Laid-Open Patent Publication No.7-103050 only discloses that the injection valve to inject fuel isswitched to the intake passage injection valve when an abnormality isdetected in fuel injection from the in-cylinder injection valve. In thepublication, overtemperature of the catalyst is not taken intoconsideration at all.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide afuel injection control apparatus and a fuel injection control methodthat readily prevent a component provided in an exhaust passage frombeing overheated in an internal combustion engine that includes aninjection valve for injecting fuel into the cylinder and an injectionvalve for injecting fuel into the intake passage.

To achieve the foregoing and other objectives and in accordance with thepurpose of the present invention, a fuel injection control apparatus foran internal combustion engine is provided. The engine has an in-cylinderinjection valve for injecting fuel into a cylinder of the engine, anintake passage injection valve for injecting fuel into an intake passageconnected to the cylinder, and an exhaust passage connected to thecylinder. The apparatus includes a temperature determining section andan injection control section. The temperature determining sectiondetermines whether the temperature of a component located in the exhaustpassage is higher than or equal to a predetermined temperature. Theinjection control section controls the in-cylinder injection valve andthe intake passage injection valve. When the temperature of thecomponent is higher than or equal to the predetermined temperature, theinjection control section increases, without changing the total amountof fuel supplied to the cylinder, the ratio of a fuel injection amountof the intake passage injection valve to a fuel injection amount of thein-cylinder injection valve compared to a case where the temperature ofthe component is not higher than or equal to the predeterminedtemperature.

The present invention also provides a fuel injection control method foran internal combustion engine. The engine has an in-cylinder injectionvalve for injecting fuel into a cylinder of the engine, an intakepassage injection valve for injecting fuel into an intake passageconnected to the cylinder, and an exhaust passage connected to thecylinder. The method includes: determining whether the temperature of acomponent located in the exhaust passage is higher than or equal to apredetermined temperature; and increasing, without changing the totalamount of fuel supplied to the cylinder, the ratio of a fuel injectionamount of the intake passage injection valve to a fuel injection amountof the in-cylinder injection valve compared to a case where thetemperature of the component is not higher than or equal to thepredetermined temperature, when the temperature of the component ishigher than or equal to the predetermined temperature.

Other aspects and advantages of the invention will become apparent fromthe following description, taken in conjunction with the accompanyingdrawings, illustrating by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating an internal combustion engineand its control apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a block circuit diagram of the control apparatus shown in FIG.1;

FIG. 3 is a flowchart showing a procedure for controlling fuel injectionaccording to the first embodiment;

FIG. 4 is a graph showing changes of catalyst temperature over time;

FIG. 5 is a graph showing the relationship of the catalyst temperatureto the amount of fuel injected from the in-cylinder injection valve andthe amount of fuel injected from the port injection valve;

FIG. 6 is a flowchart showing a procedure for setting an injectionamount ratio;

FIG. 7 is a graph showing an example of a fuel injection ratio map thatdefines the injection amount ratio of the port injection valve inrelation to engine load;

FIG. 8 is a block circuit diagram illustrating a control apparatus of aninternal combustion engine according to a second embodiment of thepresent invention;

FIG. 9 is a flowchart showing a procedure for controlling fuel injectionaccording to the second embodiment;

FIG. 10 is a flowchart showing a procedure for setting a fuel injectionamount;

FIG. 11 is a schematic diagram illustrating an internal combustionengine and its control apparatus according to a third embodiment of thepresent invention;

FIG. 12 is a flowchart showing a procedure for controlling fuelinjection according to the third embodiment;

FIG. 13 is a schematic diagram illustrating an internal combustionengine and its control apparatus according to a fourth embodiment of thepresent invention;

FIG. 14 is a flowchart showing a procedure for controlling fuelinjection according to the fourth embodiment;

FIGS. 15(a) and 15(b) show examples of maps that are referred to forobtaining a steady temperature;

FIG. 16 is a flowchart showing a procedure for controlling fuelinjection according to a fifth embodiment; and

FIG. 17 is a flowchart showing a procedure subsequent to the procedureshown in FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will now be described withreference to the drawings.

As shown in FIG. 1, an internal combustion engine 1 of the presentembodiment is a reciprocating internal combustion engine that usesgasoline as fuel. The engine 1 is particularly applicable to vehiclessuch as passenger vehicles, buses, and trucks. The engine 1 iscontrolled by a control apparatus 10 and an electronic control unit(ECU) 30, which is separate from the control apparatus 10. The controlapparatus 10 and the ECU 30 operate in association with each other tocontrol the engine 1. The engine 1 has a cylinder 1S in which acombustion chamber 11 is defined, an intake passage 4 connected to thecombustion chamber 11, and an exhaust passage 7 connected to thecombustion chamber 11. A part of the exhaust passage 7 is formed by anexhaust manifold 8. A three-way catalyst 9 is located in the exhaustmanifold 8. A piston 5 provided in the cylinder 1S is connected via aconnecting rod 13 to a crankshaft 12, which is the output shaft for theengine 1. The connecting rod 13 converts reciprocation of the piston 5into rotation of the crankshaft 12.

The engine 1 also has a first injection valve and a second injectionvalve. In this embodiment, the first injection valve is an in-cylinderinjection valve 3 that injects fuel F into the cylinder 1S, or into thecombustion chamber 11, and the second injection valve is an intakepassage injection valve 2 that injects fuel F into the intake passage 4.The joint between the combustion chamber 11 and the intake passage 4form an intake port 4 a. The intake passage injection valve 2 injectsfuel F toward the intake port 4 a. Accordingly, the intake passageinjection valve 2 will hereinafter be referred to as port injectionvalve. The port injection valve 2 and the in-cylinder injection valve 3receive fuel having a predetermined pressure through a fuel supplymechanism (not shown). The pressure of fuel supplied to the in-cylinderinjection valve 3 is higher than the pressure of fuel supplied to theport injection valve 2. Fuel F is supplied to the cylinder 1S by atleast one of the port injection valve 2 and the in-cylinder injectionvalve 3.

Air that is guided to the cylinder 1S through the intake passage 4 formsair-fuel mixture with fuel injected from the port injection valve 2 orthe in-cylinder injection valve 3. The air-fuel mixture is ignited by anignition plug 6 in the cylinder 1S to be combusted, and then becomescombustion gas. The timing for igniting the air-fuel mixture by theignition plug 6 is adjusted by an igniter 6 a provided in an upperportion of the ignition plug 6. The combustion pressure of thecombustion gas is transmitted to the piston 5, thereby reciprocating thepiston 5. After driving the piston 5, the combustion gas is guided tothe three-way catalyst 9 through the exhaust passage 7. The three-waycatalyst 9 reduces CO, HC, and NOx components in the combustion gas topurify the combustion gas.

In the present embodiment, various fuel injection modes can be performedusing two types of injection valves, namely the port injection valve 2and the in-cylinder injection valve 3. For example, when only the portinjection valve 2 is used for injecting fuel, homogeneous air-fuelmixture is formed in the cylinder 1S and such mixture is combusted(homogeneous combustion). The fuel injection mode using the portinjection valve 2 is performed, for example, when the engine 1 isoperating under relatively low load. On the other hand, when only thein-cylinder injection valve 3 is used for injecting fuel, the engine 1is operated in a combustion mode selected from a stratified combustionmode and a homogeneous combustion mode. In the stratified combustionmode, fuel is injected from the in-cylinder injection valve 3 during thecompression stroke of the engine 1 so that relatively rich air-fuelmixture is formed in the vicinity of the ignition plug 6. In this state,the mixture is ignited to perform combustion. The stratified combustionmode is performed, for example, when the engine 1 is operating underrelatively low load. In the homogenous combustion mode, fuel is injectedfrom the in-cylinder injection valve 3 during the intake stroke of theengine 1 so that homogenous air-fuel mixture is formed in the cylinder1S. In this state, the mixture is ignited to perform combustion. Thehomogenous combustion mode is performed, for example, when the engine 1is operating under relatively high load. Also, a fuel injection mode inwhich fuel is injected from both of the in-cylinder injection valve 3and the port injection valve 2 can be performed. A fuel injection modeto be performed may be selected as necessary according to theoperational state of the engine 1, such as the engine load KL and theengine rotational speed NE.

Means for detecting a parameter related to the catalyst temperature (thecatalyst bed temperature) is attached to the three-way catalyst 9. Inthis embodiment, a temperature sensor 40 that detects the temperature Tcof the catalyst bed (hereinafter, referred to as catalyst temperatureTc) is attached to the three-way catalyst 9. The detected catalysttemperature Tc is used for determining an overtemperature (OT) of thethree-way catalyst 9 and for executing control for suppressing the OT ofthe three-way catalyst 9. An air-fuel ratio sensor (hereinafter,referred to as A/F sensor) 41 is provided in the exhaust manifold 8 todetect the air-fuel ratio A/F of air-fuel mixture formed in the cylinder1S. The detected air-fuel ratio A/F is used for, for example, detectingan abnormality of combustion in the engine 1. For example, the output ofthe A/F sensor 41 is compared with a target air-fuel ratio determinedaccording to the operational state of the engine 1 to determine whetherthere is a combustion abnormality in the engine 1. In this embodiment,the output of the temperature sensor 40 and the output of the A/F sensor41 are sent to the control apparatus 10 via the ECU 30. However, theoutput may be sent directly to the control apparatus 10.

In addition to the catalyst bed temperature, parameters related to thecatalyst temperature include, for example, the temperature of exhaustgas. The exhaust gas temperature may be directly detected by a sensor ormay be estimated from a parameter other than the exhaust gastemperature. Alternatively, a map defining the relationship of thecatalyst temperature Tc to the engine operational state such as anengine load KL, an engine rotational speed NE, the air-fuel ration A/F,and an intake air flow rate GA may be prepared, and the catalysttemperature Tc that corresponds to the current engine operational statemay be obtained by referring to the map. Alternatively, a map may beprepared defining the relationship between the intake air flow rate GAand the amount of change of exhaust gas temperature per unit time, and,by referring to the map, exhaust gas temperature that corresponds to thecurrent engine operational state may be estimated. In this case, thecatalyst temperature Tc is estimated based on the estimated exhaust gastemperature. That is, even if the catalyst temperature Tc is notdirectly detected, any parameter that permits the catalyst temperatureTc to be estimated may be used as a parameter related to the catalysttemperature Tc.

As shown in FIG. 2, the control apparatus 10 includes a processingsection 10 p and a memory section 10 m. The processing section 10 pincludes a combustion determining section 21, a catalyst temperaturedetermining section 22, and an injection control section 23. The memorysection 10 m, the combustion determining section 21, the catalysttemperature determining section 22, and the injection control section 23are connected to each other with an input-output interface (I/O) 29 andperform two-way data transmission. As necessary, one-way datatransmission may be performed.

The control apparatus 10 and the ECU 30 are connected to each other withthe input-output interface 29 and performs two-way data transmission.The control apparatus 10 obtains via the ECU 30 various informationnecessary for controlling the engine 1, for example, informationrepresenting the engine operational state such as the load KL of theengine 1 and the rotational speed NE of the engine 1, and informationobtained with various sensors. The control apparatus 10 is also capableof causing the engine control executed by itself to interrupt an enginecontrol routine executed by the ECU 30. The control apparatus 10 may beconfigured by some functions of the ECU 30 when it controls the engine 1or may be incorporated in the ECU 30.

The memory section 10 m stores various programs and various datanecessary for controlling the engine 1. The memory section 10 m may be avolatile memory such as random access memory (RAM), a nonvolatile memorysuch as a flash memory, or a combination of these. The processingsection 10 p may composed of a computer that includes memory and a CPU.In this case, the combustion determining section 21, the catalysttemperature determining section 22, and the injection control section 23of the processing section 10 p correspond to functions performed by thecomputer according to control programs stored in the memory section 10m.

Alternatively, instead of performing the control programs, the controlsection 10 p may use a dedicated hardware to perform the functions ofthe combustion determining section 21, the catalyst temperaturedetermining section 22, and the injection control section 23.

A throttle sensor 42 detects the opening degree of a throttle valvelocated in the intake passage 4. An airflow sensor 43 detects an intakeair flow rate GA in the intake passage 4. A crank sensor 44 detects therotational phase (crank angle) of the crankshaft 12 and the enginerotational speed NE. A pedal depression degree sensor 45 detects thedepression degree of an acceleration pedal. The ECU 30 obtains theoutput of various sensors including these sensors 42, 43, 44, 45,thereby controlling the operation of the engine 1.

FIG. 3 is a flowchart showing a procedure for controlling fuel injectionexecuted by the control apparatus 10 and the ECU 30. When performing theprocedure, it is assumed that the engine 1 is operating using at leastthe in-cylinder injection valve 3.

At step S101, the ECU 30 obtains information such as the enginerotational speed NE and the intake air flow rate GA, thereby computingthe engine load KL and other values. Based on the engine load KL, theengine rotational speed NE and other values, the ECU 30 or the injectioncontrol section 23 computes a total fuel injection amount TAU, which isthe total amount fuel supplied to the cylinder 1S.

Next, at step S102, the catalyst temperature determining section 22compares the catalyst temperature Tc with a predetermined referencevalue, and determines whether the catalyst temperature Tc is higher thanor equal to the reference temperature. The reference value is used fordetermining whether the three-way catalyst 9 is in an overtemperaturecondition or in a condition close to the overtemperature condition, andmay be an upper temperature limit Tl of the three-way catalyst 9 or acorrected upper temperature limit Tlc (Tlc=Tl−δT), which is a valueobtained by subtracting a safety temperature margin δT from the uppertemperature limit Tl. In this embodiment, the corrected uppertemperature limit Tlc is used as the reference value.

Also, instead of comparing the catalyst temperature Tc with a referencevalue, time tn that is required for the catalyst temperature Tc tosurpass the reference value may be estimated based on the increase rateof the catalyst temperature Tc, and the time tn may be compared with apredetermined reference time. FIG. 4 is a graph showing changes of thecatalyst temperature Tc over time. For example, the catalyst temperaturedetermining section 22 may compute an increase rate ΔTc/Δt of thecatalyst temperature Tc, and estimates time tn required for the currentcatalyst temperature Tc to surpass a predetermined reference value (theupper temperature limit Tl in this case) based on the current catalysttemperature Tc and the increase rate ΔTc/Δt. When the catalysttemperature determining section 22 determines that the estimated time tnis less than or equal to a predetermined time, the procedure proceeds tostep S103.

At step S102, if the catalyst temperature Tc is lower than the correctedupper temperature limit Tlc, it is determined that the catalysttemperature Tc is normal and that there is no abnormal temperatureincrease in the three-way catalyst 9. Thus, normal fuel injection isperformed at step S108. That is, fuel is injected without changing thetotal fuel injection amount TAU and without changing the ratio betweenthe fuel injection amount from the port injection valve 2 and the fuelinjection amount from the in-cylinder injection valve 3.

On the other hand, if the catalyst temperature Tc is higher than orequal to the corrected upper temperature limit Tic at step S102, it isdetermined that the catalyst temperature Tc has substantially reachedthe upper temperature limit TI of the three-way catalyst 9. One of thecauses of increase in the catalyst temperature Tc is a high loadoperation of the engine 1 over an extended period of time. A malfunctionof the in-cylinder injection valve 3 and deposit collected on thein-cylinder injection valve 3 can cause poor formation of fuel spray.This in tern increases the air-fuel ratio A/F, or forms lean air-fuelmixture. In such a case, the catalyst temperature Tc is likely toincrease. If the engine 1 continues operating with the catalysttemperature Tc being higher than or equal to the corrected uppertemperature limit Tlc, an overtemperature condition of the three-waycatalyst 9 can degrade the exhaust gas purifying performance or thedurability of the three-way catalyst 9.

When the outcome of step S102 is positive, the injection control section23 executes control for lowering the catalyst temperature Tc.Specifically, at step S103, the injection control section 23 increasesthe ratio of the fuel injection amount from the port injection valve 2without changing the total fuel injection amount TAU, which is the totalamount of fuel to be supplied to the cylinder 1S, thereby reducing theratio of the fuel injection amount from the in-cylinder injection valve3. The ratio between the fuel injection amounts of the injection valves2, 3 is determined according to the procedure of the flowchart shown inFIG. 6, which will be discussed below. At step S104, the port injectionvalve 2 and the in-cylinder injection valve 3 are controlled to injectfuel at the determined injection amount ratio.

At step S105, the catalyst temperature determining section 22 determineswhether the catalyst temperature Tc is higher than or equal to thecorrected upper temperature limit Tlc. When the catalyst temperature Tcfalls below the corrected upper temperature limit Tlc, the procedureproceeds to step S108, at which the normal injection is performed.

FIG. 5 is a graph showing the relationship of the catalyst temperatureTc to the fuel injection amount ratio between the in-cylinder injectionvalve 3 and the port injection valve 2. As shown in FIG. 5, the catalysttemperature Tc is lowered as the fuel injection amount ratio of the portinjection valve 2 is increased. That is, the port injection promotesmixing of fuel with air more than the in-cylinder injection, and thuspromotes vaporization of fuel. As the fuel vaporization is promoted, theamount of HC and CO in exhaust gas is reduced. This suppresses thegeneration of heat due to the reaction of the catalyst 9 to HC and CO.As a result, the catalyst temperature Tc is lowered. Therefore, as longas the total fuel injection amount TAU is constant, the catalysttemperature Tc is effectively lowered by increasing the fuel injectionamount ratio of the port injection valve 2 to lower the fuel injectionamount ratio of the in-cylinder injection valve 3. Further, since thecatalyst temperature Tc is lowered without increasing the total fuelinjection amount TAU, the fuel economy is not degraded.

If the air-fuel ratio A/F is increased due to insufficient formation offuel spray from the in-cylinder injection valve 3, and the catalysttemperature Tc is increased, accordingly, the fuel injection amountratio of the port injection valve 2 is increased so that the catalysttemperature Tc is effectively lowered. Also, since the increased airfuel ratio A/F is caused to seek the target air-fuel ratio by increasingthe fuel injection amount ratio of the port injection valve 2, theconcentration of HC and CO in exhaust gas is prevented from increasing.

On the other hand, when the outcome of step S105 is positive, that is,when the catalyst temperature Tc stays equal to or above the correctedupper temperature limit Tlc even after the fuel injection amount ratioof the port injection valve 2 is increased, the injection controlsection 23 increases the total fuel injection amount TAU at step S106.At step S107, the port injection valve 2 and the in-cylinder injectionvalve 3 are controlled to inject fuel the amount of which corresponds tothe increased total fuel injection amount TAU. As the total fuelinjection amount TAU is increased, the air-fuel ratio A/F is lowered andthe mixture is richened. This improves the combustion state and thusdecreases the amount of unburned HC in exhaust gas. As a result, theexhaust gas temperature is lowered, and the catalyst temperature Tc islowered.

When increasing the total fuel injection amount TAU, the increase rateof the fuel injection amount from the port injection amount 2 may bemade greater than that of the in-cylinder injection valve 3.Alternatively, the increase of the total fuel injection amount TAU maybe entirely achieved by increase of the fuel injection amount of theport injection valve 2. Alternatively, the fuel injection amount fromthe in-cylinder injection valve 3 may be decreased to zero, and thewhole increasing amount of the total fuel injection amount TAU may beinjected only from the port injection valve 2. Accordingly, the portinjection significantly promotes the mixing of fuel with air, which inturn promotes evaporation of fuel. Thus, the amount of unburned HC inexhaust gas is effectively lowered, and the exhaust gas temperature andthe catalyst temperature Tc are lowered. It is possible to make theincrease rate of the fuel injection amount of the port injection valve 2the same as that of the in-cylinder injection valve 3. A mode forcontrolling the injection valves 2, 3 when increasing the total fuelinjection amount TAU may be set according to the engine operationalstate (including the engine rotational speed NE, the engine load KL, thefuel injection mode, the fuel injection ratio between the injectionvalves 2, 3).

If the catalyst temperature Tc is determined to be lower than thecorrected upper temperature limit Tlc at step S102 after increasing thetotal fuel injection amount TAU, the procedure proceeds to the normalinjection at step S108. On the other hand, if the catalyst temperatureTc is higher than or equal to the corrected upper temperature limit Tlc,the procedure from step S103 to step S107 is repeated until the catalysttemperature Tc falls below the corrected upper temperature limit Tlc.When the decrease rate of the catalyst temperature Tc is lower than apredetermined value, the control apparatus 10 may increase the totalfuel injection amount TAU, thereby causing at least the port injectionvalve 2 to inject fuel, that is, the control apparatus 10 may repeat theprocedure of steps 106 and 107. In this case, the catalyst temperatureTc is quickly lowered due to the decrease in the air-fuel ratio A/F.Thus, the durability of the three-way catalyst 9 is prevented from beingdegraded by a high temperature.

Next, a procedure for determining the fuel injection amount ratiobetween the port injection valve 2 and the in-cylinder injection valve 3will be described. The injection control section 23 determines the fuelinjection amount ratio between the port injection valve 2 and thein-cylinder injection valve 3 according to the procedure shown in theflowchart of FIG. 6. First, at step S201, the injection control section23 obtains the total fuel injection amount TAU from the ECU, whichamount TAU is computed by the ECU 30 based on the engine operationalstate including the engine load KL and the engine rotational speed NE.The injection control section 23 may obtain the engine operational stateincluding the engine load KL and the engine rotational speed NE from theECU 30, and compute the total fuel injection amount TAU based on theobtained information.

At the subsequent step S202, the injection control section 23 determineswhether the total fuel injection amount TAU needs to be increased. Acase where the total fuel injection amount TAU needs to be increasedrefers to a case where the catalyst temperature Tc will not besufficiently lowered by increasing the fuel injection amount ratio ofthe port injection valve 2. This corresponds to a case where the outcomeof step S105 of FIG. 3 is positive. When determining that the total fuelinjection amount TAU needs to be increased, the injection controlsection 23 proceeds to step S203. At step S203, the injection controlsection 23 adds a fuel increase amount ay to the total fuel injectionamount TAU obtained at step S201, and sets the resultant as the finalvalue of the total fuel injection amount TAU. On the other hand, whendetermining that the total fuel injection amount TAU does not need to beincreased, the injection control section 23 proceeds to step S204. Atstep S204, the injection control section 23 sets the total fuelinjection amount TAU obtained at step S201 as the final value of thetotal fuel injection amount TAU.

At the subsequent step S205, the combustion determining section 21determines whether the engine operational state is in an operation rangein which the homogeneous combustion is performed (homogenous combustionregion). If the engine operational state is in the homogeneouscombustion region, the injection control section 23 at step S206determines a ratio K of the amount fuel injected from the port injectionvalve 2 to the total fuel injection amount TAU (hereinafter, simplyreferred to as injection amount ratio K).

FIG. 7 is a graph showing an example of a fuel injection ratio map thatsets the injection amount ratio K of the port injection valve 2 to theengine load KL. As shown in FIG. 7, the injection amount ratio K is setto increase as the engine load KL increases. The injection amount ratiomap 50 is stored in the memory section 10 m of the control apparatus 10in advance. The injection control section 23 obtains the injectionamount ratio K by referring to the injection amount ratio map 50 of FIG.7 based on the engine load KL obtained through the ECU 30. When settingthe injection amount ratio map 50, parameters indicating the engineoperational state other than the engine load KL (for example, the enginerotational speed NE) may be taken into consideration.

At the subsequent step S207, the injection control section 23 determineswhether the injection amount ratio of the port injection valve 2 needsto be increased. A case where the injection amount ratio of the portinjection valve 2 needs to be increased refers to a case where thecatalyst temperature Tc becomes higher than or equal to a predeterminedreference value (the corrected upper temperature limit Tlc) when theengine 1 is operating using at least the in-cylinder injection valve 3.This corresponds to a case where the outcome of step S102 of FIG. 3 ispositive. When determining that the injection amount ratio of the portinjection valve 2 needs to be increased, the injection control section23 proceeds to step S208. At step S208, the injection control section 23adds a ratio increase amount β to the injection ratio K determined atstep S206, and sets the resultant as the final value of the injectionamount ratio K. On the other hand, when determining that the injectionamount ratio of the port injection valve 2 does not need to beincreased, the injection control section 23 proceeds to step S209. Atstep S209, the injection control section 23 sets the injection amountratio K determined at step S206 as the final value of the injectionamount ratio K.

After determining the injection amount ratio K, the injection controlsection 23 at step S210 determines a fuel injection amount Qp of theport injection valve 2 and a fuel injection amount Qd of the in-cylinderinjection valve 3 using the injection amount ratio K according to thefollowing equation.Qp=K×TAUQd=(1−K)×TAU

Therefore, when the engine operational state is in the homogeneouscombustion region, the port injection valve 2 and the in-cylinderinjection valve 3 inject fuel in accordance with the fuel injectionamounts Qp, Qd determined at step S210 at steps S104, S107, and S108.

On the other hand, when the engine operational state is determined to beout of the homogenous combustion region at step S205, in other words,when the engine operational state is determined to be in an operationalrange where the stratified combustion is performed (stratifiedcombustion region), the procedure proceeds to step S211. At step S211,the injection control section 23, as at step S207, determines whetherthe injection amount ratio of the port injection valve 2 needs to beincreased. When the injection amount ratio of the port injection valve 2does not need to be increased, the injection control section 23 proceedsto step S212. At step S212, the injection control section 23 sets thefuel injection amount Qp of the port injection valve 2 to zero and setsthe fuel injection amount Qd of the in-cylinder injection valve 3 to thetotal fuel injection amount TAU, so that the engine 1 operates with fuelinjection from only the in-cylinder injection valve 3.

On the other hand, when the injection amount ratio of the port injectionvalve 2 needs to be increased, the injection control section 23increases the injection amount ratio of the port injection valve 2 atstep S213. In this embodiment, the fuel injection amount Qp of the portinjection valve 2 is set to the total fuel injection amount TAU, and thefuel injection amount Qd of the in-cylinder injection valve 3 is set tozero. That is, if the three-way catalyst 9 is not in an overtemperaturecondition when the engine operational state is in the stratifiedcombustion region, the stratified combustion is performed with fuel ofthe total injection amount TAU injected from the in-cylinder injectionvalve 3. If the three-way catalyst 9 is in an overtemperature condition,the homogeneous combustion is performed with fuel of the total injectionamount TAU injected from the port injection valve 2.

In this manner, even if the engine operational state is in thestratified combustion region, fuel is injected from the port injectionvalve 2 so that mixing of fuel and air is promoted, which promotesevaporation of fuel. As a result, the amount of unburned HC in exhaustgas is reduced, and the exhaust gas temperature and the catalysttemperature Tc are lowered. Even if the engine operational state is inthe stratified combustion region, the injection amount ratio of the portinjection valve 2 may be changed according to the engine operationalstate such as the engine load KL and the engine rotational speed NE asin the homogeneous combustion region. For example, if the catalysttemperature becomes higher than or equal to a predetermined referencevalue when the engine operational state is in the stratified combustionregion, the injection amount ratio of the port injection valve 2 may beincreased as the engine load KL is increased.

As described above, in this embodiment, whether the catalyst temperatureTc becomes higher than or equal to a predetermined reference value (thecorrected upper temperature limit Tlc) is determined based on aparameter related to the catalyst temperature Tc. If the catalysttemperature Tc becomes higher than or equal to the reference value, thefuel injection amount from the port injection valve 2 is increasedcompared to the case where the catalyst temperature Tc is lower than thereference value. Therefore, an overtemperature condition of thethree-way catalyst 9 is reliably and accurately detected, and thecatalyst temperature Tc is readily lowered to reliably prevent thethree-way catalyst 9 from being overheated.

Further, in the present embodiment, when the catalyst temperature Tcbecomes higher than or equal to the reference value, the fuel injectionamount ratio of the port injection valve 2 is first increased withoutchanging the total fuel injection amount TAU. Therefore, the catalysttemperature Tc is reliably lowered while preventing the fuel economyfrom deteriorating and while limiting the influence to the air-fuelratio A/F.

For example, when the catalyst temperature Tc increases due toinsufficient formation of fuel spray from the in-cylinder injectionvalve 3, fuel is injected from the port injection valve 2 so that theair-fuel ratio A/F is prevented from increasing (air-fuel mixture isprevented from being lean). Accordingly, the air-fuel ratio A/F ismaintained at the target value, and the overtemperature of the catalyst9 is suppressed.

The configuration of the present embodiment is applicable to either of acase where the engine operational state is in the homogenous combustionregion or in the stratified combustion region. However, since iteffectively lowers the catalyst temperature, the configuration isparticularly suitable for a high load and high output operation(homogeneous combustion region), in which the catalyst temperature islikely to increase. Further, the configuration of the present embodimentis applicable to other embodiments, which will be discussed below.

A second embodiment of the present invention will now be described withreference to FIGS. 8 and 10. The differences from the first embodimentof FIGS. 1 to 7 will mainly be discussed. This embodiment is differentfrom the first embodiment mainly in the following points. That is, inthis embodiment, when the port injection valve and the in-cylinderinjection valve 3 are both-injecting fuel, an actual air-fuel ratio AF1is compared with a target air-fuel ratio AFa prior to the determinationof the catalyst temperature Tc. Then, the fuel injection amount of theport injection valve 2 or the in-cylinder injection valve 3 is increasedsuch that the actual air-fuel injection valve AF1 seeks the targetair-fuel ratio AFa. Like or the same reference numerals are given tothose components that are like or the same as the correspondingcomponents of the first embodiment and detailed explanations areomitted. Refer to FIG. 1 as necessary.

As shown in FIG. 8, a processing section 10 p′ of a control apparatus10′ according to the present embodiment includes an air-fuel ratiodetermining section 24.

A fuel injection controlling procedure according to the presentembodiment will now be described with reference to the flowchart of FIG.9. First, at step S301, the ECU 30 obtains information such as theengine rotational speed NE and the intake air flow rate GA, therebycomputing the engine load KL and other values. Based on the engine loadKL, the engine rotational speed NE, and other parameters, the total fuelinjection amount TAU is computed.

At step S302, the air-fuel ratio determining section 24 compares theair-fuel ratio A/F obtained from the output of the A/F sensor 41, or theactual air-fuel ratio AF1 during the operation of the engine 1, with thetarget air-fuel ratio AFa. When the actual air-fuel ratio AF1 is morethan the target air-fuel ratio AFa, that is, when the engine 1 isoperating with lean air-fuel mixture, it is assumed that there is amalfunction such as insufficient formation of fuel spary in at least oneof the port injection valve 2 and the in-cylinder injection valve 3,causing abnormal combustion or unstable combustion.

Thus, if the actual air-fuel ratio AF1 is more than the target air-fuelratio AFa at step S302, it is assumed that there is an abnormality inthe in-cylinder injection valve 3. This is because since fuel of higherpressure is supplied to the in-cylinder injection valve 3 compared tothe port injection valve 2, the possibility of malfunctioning is higherin the in-cylinder injection valve 3 than in the port injection valve 2.Also, since the in-cylinder injection valve 3 injects fuel into thecylinder 1S, in which combustion takes place, foreign matter such ascarbon deposits is likely to collect on the in-cylinder injection valve3. For these reasons, if the actual air-fuel ratio AF1 is more than thetarget air-fuel ratio AFa at step S302, the injection control section 23increases the fuel injection amount Qp of the port injection valve 2 atstep S303. Specifically, the injection control section 23 adds apredetermined fuel increase amount y to the fuel injection amount Qp ofthe port injection valve 2 in the total fuel injection amount TAU andsets the resultant (Qp+γ) as a corrected fuel injection amount Qp1 ofthe port injection valve 2. At step S304, the port injection valve 2injects fuel the amount of which corresponds to the corrected fuelinjection amount Qp1.

At the subsequent step S305, the air-fuel ratio determining section 24obtains the actual air-fuel ratio AF1 from the A/F sensor 41 again, andcompares it with the target air-fuel ratio AFa. If the outcome of stepS305 is negative, that is, if the actual air-fuel ratio AF1 is less thanor equal to the target air-fuel ratio AFa, the catalyst temperaturedetermining section 22 proceeds to step S308 and compares the catalysttemperature Tc with a predetermined reference value. In the presentembodiment, the reference value is a catalyst map temperature Tcm thatis set with respect to the catalyst temperature to determine whether thethree-way catalyst 9 is in an overtemperature condition.

If the actual air-fuel ratio AF1 becomes less than or equal to thetarget air-fuel ratio AFa at step S305 after the port injection valve 2injects fuel the amount of which corresponds to the corrected fuelinjection amount Qp1, which is an increased fuel injection amount, it isassumed that there is an abnormality in the in-cylinder injection valve3. The assumption result is preferably stored in the memory of the ECU30 so that the cause of the abnormality is easily identified duringmaintenance. Also, it may be configured that when the number of increaseof the fuel injection amount of the port injection valve 2 reaches apredetermined number, notice may be served to encourage the driver toperform maintenance.

If the outcome of step S305 is positive, that is, if the actual air-fuelratio AF1 is more than the target air-fuel ratio AFa despite increase ofthe fuel injection amount from the port injection valve 2, it is assumedthat there is also an abnormality in the port injection valve 2. In thiscase, the injection control section 23 increases the fuel injectionamount Qd of the in-cylinder injection valve 3 at step S306.Specifically, the injection control section 23 adds a predetermined fuelincrease amount δ to the fuel injection amount Qd of the in-cylinderinjection valve 3 in the total fuel injection amount TAU and sets theresultant (Qd+δ) as a corrected fuel injection amount Qd1 of thein-cylinder cylinder injection valve 3. At step S307, the in-cylinderinjection valve 3 injects fuel the amount of which corresponds to thecorrected fuel injection amount Qd1. As in the case of the in-cylinderinjection valve 3, if it is assumed that there is an abnormality in theport injection valve 2, the assumption result is preferably stored inthe memory of the ECU 30.

FIG. 10 is a flowchart showing a procedure for determining a fuelinjection amount. The injection control section 23 determines the fuelinjection amount of the port injection valve 2 and the in-cylinderinjection valve 3 according to the procedure shown in the flowchart ofFIG. 10. First, at step S401, the injection control section 23determines ratio between the fuel injection amount of the port injectionvalve 2 and the fuel injection amount of the in-cylinder injection valve3 based on the engine operational state and other factors. Then, basedon the ratio and the total fuel injection amount TAU obtained from theengine load KL, the engine rotational speed NE, and other parameters,the injection control section 23 determines the fuel injection amount Qpof the port injection valve 2 and the fuel injection amount Qd of thein-cylinder injection valve 3.

At step S420, the injection control section 23 adds a predetermined fuelincrease amount γ to the fuel injection amount Qp and sets the resultant(Qp+γ) as a corrected fuel injection amount Qp1 of the port injectionvalve 2. The corrected fuel injection amount Qp1 is used for increasingthe fuel injection amount Qp of the port injection valve 2 at step S303of FIG. 9. Next, at step S403, the injection control section 23 adds apredetermined fuel increase amount δ to the fuel injection amount Qd andsets the resultant (Qp+δ) as a corrected fuel injection amount Qd1 ofthe in-cylinder injection valve 3. The corrected fuel injection amountQd1 is used for increasing the fuel injection amount Qd of thein-cylinder injection valve 3 at step S306 of FIG. 9. If the fuelinjection amount of the injection valves 2, 3 is increased, thesubsequent control is executed using the increased fuel injection amountas a reference value.

Referring back to FIG. 9, whether the actual air-fuel ratio AF1 is morethan the target air-fuel ratio AFa is determined again at step S302subsequent to step S307. If the actual air-fuel ratio AF1 is more thanthe target air-fuel ratio AFa, steps S303 to S307 are repeated until theactual air-fuel ratio AF1 becomes less than or equal to the targetair-fuel ratio AFa, and the injection amounts Qp, Qd are increased bythe fuel increase amounts γ, δ, respectively. When the actual air-fuelratio AF1 becomes less than or equal to the target air-fuel ratio AFa,the catalyst temperature Tc is compared with the catalyst maptemperature Tcm, which is a predetermined reference value, at step S308.

The process of steps S308 to S314 is substantially the same as theprocess of steps S102 to S108 of FIG. 3 except that the catalyst maptemperature Tcm is used the predetermined reference value instead of thecorrected upper temperature limit Tlc.

If the catalyst temperature Tc is lower than the catalyst maptemperature Tcm at step S308, the procedure proceeds to the normalinjection of step S314. When steps S303 to S307 are executed, the fuelinjection amounts of the injection valves 2, 3 in the normal injectionare increased fuel injection amounts. On the other hand, if the catalysttemperature Tc is higher than or equal to the catalyst map temperatureTcm at step S308, it means that the three-way catalyst 9 is in anovertemperature condition despite the fact that the actual air-fuelratio AF1 is normal. Operating the engine 1 in this state reduces thedurability of the three-way catalyst 9. Thus, if the outcome of stepS308 is positive, the injection control section 23 proceeds to stepS309. At step S309, the injection control section 23 increases the fuelinjection amount ratio of the port injection valve 2 without changingthe total fuel injection amount TAU, thereby reducing the fuel injectionamount ratio of the in-cylinder injection valve 3. At step S310, theport injection valve 2 and the in-cylinder injection valve 3 inject fuelat the determined injection amount ratio. The injection amount ratio ofthe port injection valve 2 is increased because, as described above, itis effective to lower the catalyst temperature Tc.

If the catalyst temperature Tc is determined to be lower than thecatalyst map temperature Tcm at step S311, the procedure proceeds tostep S314, at which the normal injection is performed. In this manner,the catalyst temperature Tc is lowered while suppressing deteriorationof the fuel economy. Further, since actual air-fuel ratio AF1 is causedto seek the target air-fuel ratio AFa, HC and CO in exhaust gas areprevented from increasing.

On the other hand, when the outcome of step S311 is positive, that is,when the catalyst temperature determining section 22 determines that thecatalyst temperature Tc stays equal to or above the catalyst maptemperature Tcm even after the fuel injection amount ratio of the portinjection valve 2 is increased, the catalyst temperature determiningsection proceeds to step S312. At step S312, the injection controlsection 23 increases the total fuel injection amount TAU. At step S313,the port injection valve 2 and the in-cylinder injection valve 3 arecontrolled to inject fuel the amount of which corresponds to theincreased total fuel injection amount TAU. As a result, the exhaust gastemperature is lowered, and the catalyst temperature Tc is lowered.

If the catalyst temperature Tc is determined to be lower than thecatalyst map temperature Tcm at step S308 after increasing the totalfuel injection amount TAU, the procedure proceeds to step S314, at whichthe normal injection is performed. On the other hand, if the catalysttemperature Tc is higher than or equal to the catalyst map temperatureTcm, the procedure from step S309 to step S313 is repeated until thecatalyst temperature Tc falls below the catalyst map temperature Tcm. Inthis manner, the catalyst temperature Tc is lowered below the catalystmap temperature Tcm, so that the durability of the three-way catalyst 9is not reduced.

A third embodiment of the present invention will now be described withreference to FIGS. 11 and 12. In the present embodiment, an engine 1 hastwo three-way catalysts 9A and 9B. Each of the three-way catalysts 9Aand 9B corresponds to two of four cylinders 1SA, 1SB, 1SC, and 1SD. Wheneither one of the three-way catalyst 9A, 9B is in an overtemperaturecondition, fuel injection of cylinders corresponding the catalyst in anovertemperature condition is controlled to lower the temperature of thecatalyst.

As shown in FIG. 11, the engine 1 is an in-line four-cylinder engine,and includes the first three-way catalyst 9A and the second three-waycatalyst 9B. First to fourth exhaust passages 8A to 8D each extend fromone of the four cylinders 1SA to 1SD. Exhaust gas from the first andfourth cylinders 1SA and 1SD is conducted to the first three-waycatalyst 9A through the corresponding exhaust passages 8A, 8D and ispurified by the first three-way catalyst 9A. Exhaust gas from the secondand third cylinders 1SB and 1SC is conducted to the second three-waycatalyst 9B through corresponding exhaust passages 8B, 8C and ispurified by the second three-way catalyst 9B.

The temperature of the first three-way catalyst 9A (first catalysttemperature Tc1) is detected by a first temperature sensor 40A, and thetemperature of the second three-way catalyst 9B (second catalysttemperature Tc2) is detected by a second temperature sensor 40B. A firstport injection valve 2A and a first in-cylinder injection valve 3Acorrespond to the first cylinder 1SA. A second port injection valve 2Band a second in-cylinder injection valve 3B correspond to the secondcylinder 1SB. A third port injection valve 2C and a third in-cylinderinjection valve 3C correspond to the third cylinder 1SC. A fourth portinjection valve 2D and a fourth in-cylinder injection valve 3Dcorrespond to the fourth cylinder 1SD. Although the engine 1 has the twothree-way catalysts 9A, 9B in the example of FIG. 11, the engine 1 mayhave three or more three-way catalysts. The number of cylinders of theengine 1 is not limited four, but may be any number greater than one.

The basic configuration of the control apparatus 10 that controls theengine 1 is basically the same as that shown in FIG. 2. Therefore, referto FIG. 2 as necessary for the description of the configuration of thecontrol apparatus 10.

FIG. 12 is a flowchart showing a procedure of fuel injection controlaccording to this embodiment. When performing the procedure, it isassumed that the engine 1 is operating using at least the first tofourth in-cylinder injection valves 3A to 3D.

First, at step S501, the ECU 30 obtains information such as the enginerotational speed NE and the intake air flow rate GA, thereby computingthe engine load KL and other values. Based on the engine load KL, theengine rotational speed NE and other values, the ECU 30 or the injectioncontrol section 23 computes the total fuel injection amount TAU.

Next, at step S502, the catalyst temperature determining section 22compares the first catalyst temperature Tc1 obtained from the firsttemperature sensor 40A with the second catalyst temperature Tc2 obtainedfrom the second temperature sensor 40B. If the outcome of step S502 ispositive, that is, if the second catalyst temperature Tc2 is higher thanthe first catalyst temperature Tc1, the catalyst temperature determiningsection 22 proceeds to step S503. At step S503, the catalyst temperaturedetermining section 22 determines whether the second catalysttemperature Tc2 is higher than or equal to the corrected uppertemperature limit Tlc. As in the flowchart of FIG. 3, the correctedupper temperature limit Tlc (Tlc=Tl−δT) is used as a reference value fordetermining the catalyst temperature in this embodiment.

If the second catalyst temperature Tc2 is lower than the corrected uppertemperature limit Tlc, it is determined that the second catalysttemperature Tc2 is normal and that there is no abnormal temperatureincrease in the second three-way catalyst 9B. Thus, the procedureproceeds to step S508, at which the normal fuel injection is performed.On the other hand, if the second catalyst temperature Tc2 is higher thanor equal to the corrected upper temperature limit Tlc, it is determinedthat the second catalyst temperature Tc2 has substantially reached theupper temperature limit Tl of the second three-way catalyst 9B.Therefore, if the engine 1 continues operating in this state, anovertemperature condition of the second three-way catalyst 9B degradesthe performance and the durability of the second three-way catalyst 9B.

When the outcome of step S503 is positive, the injection control section23 executes control for lowering the second catalyst temperature Tc2.Specifically, the injection control section 23 increases the ratio ofthe fuel injection amount from the second and third port injectionvalves 2B and 2C without changing the total fuel injection amount TAU,thereby reducing the ratio of the fuel injection amount from the secondand third in-cylinder injection valves 3B and 3C. At step S505, thesecond and third port injection valves 2B, 2C and the second and thirdin-cylinder injection valves 3B, 3C are controlled to inject fuel at thedetermined injection amount ratio.

At step S506, the catalyst temperature determining section 22 determineswhether the second catalyst temperature Tc2 is higher than or equal tothe corrected upper temperature limit Tlc. When the second catalysttemperature Tc2 falls below the corrected upper temperature limit Tlc,the procedure proceeds to step S508, at which the normal injection isperformed.

On the other hand, when the outcome of step S506 is positive, that is,when the second catalyst temperature Tc2 stays equal to or above thecorrected upper temperature limit Tlc even after the fuel injectionamount ratio of the second and third port injection valves 2B, 2C isincreased, the injection control section 23 increases the total fuelinjection amount TAU at step S507. At step S507A, the second and thirdport injection valves 2B, 2C and the second and third in-cylinderinjection valves 3B, 3C are controlled to inject fuel the amount ofwhich correspond to the increased total fuel injection amount TAU. Thislowers the air-fuel ratio A/F and richens the air-fuel mixture.Accordingly, the exhaust gas temperature and the second catalysttemperature Tc2 are lowered.

If the second catalyst temperature Tc2 is determined to be lower thanthe corrected upper temperature limit Tlc at step S503 after increasingthe total fuel injection amount TAU, the procedure proceeds to stepS508, at which the normal injection is performed. The procedure thenproceeds to step S509 so that monitoring of the first catalysttemperature Tc1 is started. On the other hand, if the second catalysttemperature Tc2 is higher than or equal to the corrected uppertemperature limit Tlc, the procedure from step S503 to step S507A isrepeated until the second catalyst temperature Tc2 falls below thecorrected upper temperature limit Tlc. When the decrease rate of thesecond catalyst temperature Tc2 is lower than a predetermined value, thecontrol apparatus 10 may increase the total fuel injection amount TAU,thereby causing at least the port injection valves 2B, 2C to injectfuel, that is, the control apparatus 10 may repeat the procedure ofsteps S507 and S507A. In this case, the second catalyst temperature Tc2is quickly lowered, and the durability of the second three-way catalyst9B is prevented from being degraded by high temperature.

If the first catalyst temperature Tc1 is determined to be higher thanthe second catalyst temperature Tc2 at step S502, it is determined thatthe second three-way catalyst 9B is not in an overtemperature condition,but is functioning normally. Thus, normal injection is performed at stepS508. At the subsequent step S509, the catalyst temperature determiningsection 22 determines whether the first catalyst temperature Tc1 ishigher than or equal to the corrected upper temperature limit Tlc. Whenthe first catalyst temperature Tc1 is determined to be lower than thecorrected upper temperature limit Tlc, the procedure proceeds to stepS514, at which the normal injection is performed. Then, the procedurereturns to step S501 and the monitoring of the first and second catalysttemperatures Tc1, Tc2 is continued.

On the other hand, if the first catalyst temperature Tc1 is determinedto be higher than or equal to the corrected upper temperature limit Tlc,the injection control section 23 executes a process for lowering thefirst catalyst temperature Tc1 at steps S510 to S513A. The process ofsteps S510 to S513A is the same as the process of the above describedsteps S504 to S507A except that the subjects of the control are thefirst and fourth port injection valves 2A, 2D and the first and fourthin-cylinder injection valves 3A, 3D. Therefore, the description of theprocess of steps S510 to S513A is omitted. The ratio between the fuelinjection amounts of the port injection valves and the in-cylinderinjection valves can be determined according to the procedure of theflowchart shown in FIG. 6.

In the present embodiment, fuel injection control for lowering thecatalyst temperature is performed only for cylinders corresponding tooverheated ones of the three-way catalysts 9A, 9B. Therefore, thetemperature of overheated catalysts is favorably lowered. Also, sincefuel is not supplied in a quantity more than necessary to cylinderscorresponding to non-overheated catalysts, the fuel economy is preventedfrom deteriorating.

A fourth embodiment according to the present invention will now bedescribed with reference to FIGS. 13 to 15(b). Like or the samereference numerals are given to those components that are like or thesame as the corresponding components of the first embodiment of FIGS. 1to 7 and detailed explanations are omitted.

As shown in FIG. 13, an engine 1 of the present embodiment has anexhaust temperature sensor 46 for detecting the temperature of exhaustgas. Reference numeral 47 represents a surge tank provided in the intakepassage 4.

An ECU 130 corresponds to the control apparatus 10 and the ECU 30 shownin FIG. 2, and is configured by a digital computer. The ECU 130 includesa CPU 136, a ROM 137, a RAM 138, an input port 139, and an output port140, which are interconnected by a bidirectional bus 135.

FIG. 14 is a flowchart showing a procedure of fuel injection controlaccording to this embodiment. When performing the procedure, it isassumed that the engine 1 is operating using at least the in-cylinderinjection valve 3. First at step S601, the ECU 130 determines whetherthe engine 1 is operating under high load. If the engine 1 is operatingunder high load, the ECU 130 sets a fuel injection mode for causing thein-cylinder injection valve 3 to inject fuel and prohibiting the portinjection valve 2 from injecting fuel. Then, the ECU 130 proceeds tostep S603. If the engine 1 is not operating under high load, the ECU 130sets an injection mode that is different from the injection mode set atstep S602, and proceeds to step S706 of FIG. 603.

At step S603, the ECU 130 determines the fuel injection mode.Specifically, the ECU 130 determines the ratio between the fuelinjection amount of the in-cylinder injection valve 3 and the fuelinjection amount of the port injection valve 2. For example, when theengine 1 is operating under high load and the in-cylinder injection isbeing performed, the fuel injection amount ratio (in-cylinder injection:port injection) is 10:0. Next at step S604, the ECU 130 estimates thetemperature of the three-way catalyst 9 (catalyst temperature Tc) usingsteady temperature maps (refer to FIGS. 15(a) and 15(b)) that correspondto the determined fuel injection mode (injection amount ratio). Themethod for estimating the catalyst temperature Tc will be describedbelow.

Subsequently, at S604, the ECU 130 determines whether the three-waycatalyst 9 is in an overtemperature condition based on the estimatedcatalyst temperature Tc. The determination can be carried out bycomparing the catalyst temperature Tc with a predetermined referencetemperature as in the first embodiment. When determining that thecatalyst 9 is not in an overtemperature condition, the ECU 130 proceedsto step S606. At step S606, the ECU 130 determines whether the estimatedtemperature Tc is higher than or equal to a predetermined temperature.The predetermined temperature is lower than a temperature (the referencevalue mentioned above) for determining whether the catalyst 9 is in anovertemperature condition. The predetermined temperature is used as athreshold value for determining whether there is a possibility that thecatalyst 9 may be in an overtemperature condition.

If the estimated catalyst temperature Tc is higher than or equal to thepredetermined temperature, the ECU 130 proceeds to step S607 and changesthe fuel combustion mode. Specifically, the ECU 130 changes the fuelinjection amount ratio between the in-cylinder injection valve 3 and theport injection valve 2 such that the ratio of the fuel injection amountratio of the port injection valve 2 to that of the in-cylinder injectionvalve 3 is increased from the current state. The total fuel injectionamount of the in-cylinder injection valve 3 and the port injection valve2, or the total amount of fuel injection supplied to the cylinder 1S, isnot changed. The ratio of the port injection amount to the in-cylinderinjection amount can be interpreted as the ratio of the port injectionamount to the total amount fuel supplied to the cylinder 1S.

Next, the ECU 130 returns to step S603 and determines the changed fuelinjection mode, or the changed fuel injection amount ratio. In thesubsequent step S604, the ECU 130 estimates the temperature of thecatalyst 9 using a map that corresponds to the changed injection amountratio. If a state continues where the catalyst 9 is determined to be notoverheated at step S605 and the catalyst temperature Tc is higher thanor equal to the predetermined temperature at step S606, the process ofsteps S603 to S607 is repeated. Accordingly, the ratio of the fuelinjection amount of the port injection valve 2 to that of thein-cylinder injection valve 3 is gradually increased. Also, every timethe injection amount ratio is changed, the catalyst temperature Tc isestimated.

For example, assume that the current injection amount ratio (in-cylinderinjection: port injection) is 9:1. In this case, if the outcome of stepS605 is negative and the outcome of step S606 is positive, the injectionamount ratio (in-cylinder injection: port injection) is changed, forexample, to 5:5 at step S607. Even while the fuel injection is performedaccording to the changed injection amount ratio, if the outcome of stepS605 is negative and the outcome of step S606 is positive, the injectionamount ratio (in-cylinder injection: port injection) is changed to 3:7at step S607.

On the other hand, if the estimated catalyst temperature Tc is lowerthan the predetermined temperature at step S606, the current process istemporarily suspended. Also, if the catalyst 9 is determined to be in anovertemperature condition at step S605, the ECU 130 increases the fuelinjection amount to prevent the catalyst 9 from being overheated andreturns to step S603. As described in the first embodiment, the increaseof the fuel injection amount may be achieved by making the increase rateof the fuel injection of the port injection valve 2 more than or equalto that of the in-cylinder injection valve 3. Alternatively, theincrease of the fuel injection amount may be entirely achieved byincrease of the fuel injection amount of the port injection valve 2.

In this manner, when the catalyst 9 is not in an overtemperaturecondition, the ratio of the port injection amount to the in-cylinderinjection amount is gradually increased. Then, when the catalyst 9enters an overtemperature condition, the fuel injection amount isincreased. In a case where increase of the fuel injection amount isrepeatedly performed, the amount of fuel increase in each increasing ispreferably decreased compared to the previous fuel increase amount.

Next, estimation of the catalyst temperature Tc performed at step S604will be described. The ECU 130 computes the engine rotational speed NEbased on an output signal from a crank sensor 44. The ECU 130 dividesthe intake air flow rate GA detected by an airflow sensor 43 by theengine operational speed NE to obtain the engine load KL (g/rev)(GN=GA/NE). The ECU 130 then estimates the catalyst temperature Tc basedon the engine rotational speed NE, the engine load KL, and the steadytemperature map that corresponds to the injection amount ratiodetermined at step S603 of FIG. 14. In the present embodiment, thecatalyst temperature Tc is obtained through a weighted average process,or “gradual-change” process. Specifically, the catalyst temperature Tc[° C.] is obtained using the following equations.Catalyst Temperature Tc[° C.]=(1−[Time Constant])×[Previous Value]+[TimeConstant]×([Steady Temperature]−[Previous Value])orCatalyst Temperature Tc[° C.]=[Previous Value]+[Time Constant]×[SteadyTemperature]

The previous value refers to the catalyst temperature Tc obtained in theprevious cycle.

If the engine 1 continues operating in a predetermined constantoperational state (the engine rotational speed NE, the engine load KL,the fuel injection mode (the ratio between the in-cylinder injectionamount and the port injection)), that is, if the stable operation of theengine 1 continues, the catalyst temperature Tc seeks a certaintemperature. The convergence value of the catalyst temperature Tc duringthe stable operation of the engine 1 is the steady temperature in theabove equation.

The time constant is a numerical value representing the rate of changeof the catalyst temperature Tc and takes on a value from zero to one.The time constant takes on a value close to one in an engine operationalstate where the catalyst temperature Tc is changed quickly. The timeconstant takes on a value close to zero in an engine operational statewhere the catalyst temperature Tc is changed slowly. The time constantis computed based on the engine operational state, such as the engineload KL, the engine rotational speed NE, and the fuel injection amountratio by referring to a predetermined time constant map.

FIGS. 15(a) and 15(b) show examples of maps that are referred to forobtaining the steady temperature. The constant temperature map of FIG.15(a) is used in a case where the injection amount ratio (in-cylinderinjection: port injection) is 10:0. The constant temperature map of FIG.15(b) is used in a case where the injection amount ratio (in-cylinderinjection: port injection) is 0:10. Other than these maps, a number ofsteady temperature maps corresponding to injection amount ratios areprepared. The time constant map and the steady temperature maps areobtained, for example, through experiments, and are stored in the ROM137 of the ECU 130. When computing the catalyst temperature Tc, the ECU130 retrieves the steady temperature map and the time constant map fromthe ROM 137. The steady temperature and the time constant may becomputed according to predetermined functions expressions instead of themaps. The catalyst temperature Tc may be computed based on a valuemeasured by the exhaust temperature sensor 46. Alternatively, thecatalyst temperature Tc may be obtained from the temperature sensor 40,which directly detects the catalyst temperature as in the firstembodiment.

In general, the amount of heat generated by combustion per unit time isincreased as the engine load and the rotational speed of the engine 1are increased. Accordingly, the catalyst temperature Tc is increased. Asshown in FIGS. 15(a) and 15(b), under a condition where the enginerotational speed NE and the engine load KL are constant, the steadytemperature is higher when the ratio of the in-cylinder injection amountis 100% than when the ratio of the in-cylinder injection amount is 0%.In other words, the catalyst temperature Tc is lower when the portinjection is performed than when the in-cylinder injection is performed.Therefore, when the catalyst temperature Tc needs to be prevented fromincreasing, it is effective to increase the ratio of the port injectionamount.

In the present invention described above, when the catalyst temperatureTc is higher than or equal to a predetermined temperature and thecatalyst 9 is not in an overtemperature condition, the ratio of the portinjection amount to the in-cylinder injection amount is increased.Therefore, as in the first embodiment, the catalyst temperature Tc isfavorably lowered while reducing toxic substances such as CO, HC, andNOx contained exhaust gas and black smoke. Also, since the catalysttemperature Tc is prevented from increasing by increasing the portinjection amount ratio, the fuel injection amount itself does not needto be increased. This prevents the fuel economy from deteriorating.

Further, when the engine 1 is operating under high load, the in-cylinderinjection, which excels in generating power of the engine 1, is activelyperformed. When the temperature of the catalyst 9 needs to be preventedfrom increasing, the port injection amount ratio is gradually increased.Accordingly, the catalyst temperature Tc is prevented from increasingwhile permitting the engine 1 to maximize the power generationperformance.

When the catalyst 9 is in an overtemperature condition, the fuelinjection amount is increased so that the catalyst temperature Tc isreliably prevented from overheating.

A fifth embodiment of the present invention will now be described withreference to FIGS. 16 and 17. The differences from the fourth embodimentof FIGS. 13 to 15(b) will mainly be discussed.

FIGS. 16 and 17 are flowcharts showing a procedure of fuel injectioncontrol according to this embodiment. As shown in FIG. 16, first at stepS701, the ECU 130 determines whether an engine 1 is operating under highload. If the engine 1 is operating under high load, the ECU 130 sets afuel injection mode for causing the in-cylinder injection valve 3 toinject fuel and prohibiting the port injection valve 2 from injectingfuel. Then, the ECU 130 proceeds to step S703. If the engine 1 is notoperating under high load, the ECU 130 sets an injection mode that isdifferent from the injection mode set at step S702. For example, the ECU130 sets a mode where only the port injection valve 2 injects fuel.Subsequently, the ECU 130 proceeds to step S706 of FIG. 17.

At step S703, the ECU 130 retrieves a steady temperature map and a timeconstant map that correspond to the fuel injection amount ratio(in-cylinder injection: port injection) of 10:0, and estimates thecatalyst temperature Tc. The estimation of the catalyst temperature Tcis performed in the same manner as the fourth embodiment, and the map ofFIG. 15(a) is used as the steady temperature map. At step S704, the ECU130 determines whether the estimated catalyst temperature Tc is higherthan or equal to a predetermined temperature. As described in the fourthembodiment, the predetermined temperature is lower than a temperature(reference value) for determining that the catalyst 9 is in anovertemperature condition. If the catalyst temperature Tc is lower thanthe predetermined temperature, the current process is temporarilysuspended.

On the other hand, if the catalyst temperature Tc is higher than orequal to the predetermined temperature, the ECU 130 proceeds to stepS705 and changes the fuel combustion mode. Specifically, the ECU 130sets a fuel injection mode in which the in-cylinder injection valve 3 isprohibited from injecting fuel and the port injection valve 2 injectsfuel. Therefore, the fuel injection amount ratio (in-cylinder injection:port injection) is 0:10.

Then, at step S706, the ECU 130 retrieves a steady temperature map and atime constant map that correspond to the fuel injection amount ratio(in-cylinder injection: port injection) of 0:10, and estimates thecatalyst temperature Tc- The map of FIG. 15(b) is used as the steadytemperature map. At step S707, the ECU 130 determines whether thecatalyst 9 is in an overtemperature condition. If the catalyst 9 is notin an overtemperature condition, the ECU 130 temporarily suspendscurrent process. When determining that the catalyst 9 is in anovertemperature condition, the ECU 130 proceeds to step S708. At stepS708, the ECU 130 increases the fuel injection amount of the in-cylinderinjection valve 3, and temporarily suspends the current process.

In this embodiment, when the catalyst 9 is determined to be in anovertemperature condition, fuel is increased through the in-cylinderinjection (step S708). That is, in a state immediately before fuel isincreased through the in-cylinder injection, fuel to be combusted issupplied only by the port injection. If the fuel injection amount isincreased in this state, the increased amount is hardly combusted.Therefore, fuel increase through the in-cylinder injection does not slowdown combustion while air is cooled by heat of evaporation of fueldirectly injected into the cylinder 1S. Therefore, fuel increase throughthe in-cylinder injection effectively lowers the exhaust gas temperatureand the catalyst temperature Tc.

In this manner, depending on the fuel injection mode prior to increaseof the fuel amount, it is effective to increase the amount of fuel byusing only the in-cylinder injection valve 3. That is, a mode forcontrolling the injection valves 2, 3 when increasing the amount of fuelmay be set according to the engine operational state (including theengine rotational speed NE, the engine load KL, the fuel injection mode,the fuel injection ratio between the injection valves 2, 3) asnecessary.

In the illustrated embodiments, the intake passage injection valve isnot limited to the port injection valve 2, which injects fuel toward theintake port 4 a. The intake passage injection valve may be an injectionvalve that is located in the surge tank 47 (see FIG. 13). For example,the intake passage injection valve may be a cold start injector, whichis actuated when the engine 1 is started cold.

Instead of detecting the intake air flow rate GA with the airflow sensor43, the intake air flow rate may be computed based on an intake airpressure detected by a pressure sensor provided in the intake passage 4.The engine load KL can be computed using the depression degree of theacceleration pedal as a parameter.

In the illustrated embodiments, prevention of overheating of thecatalyst 9 is described. However, the present invention may be appliedto prevention of overheating of other components provided in an exhaustsystem (exhaust passage 7). That is, the present invention may beapplied to exhaust system components, such as the A/F sensor 41 and theexhaust temperature sensor 46.

1. A fuel injection control apparatus for an internal combustion engine,wherein the engine has an in-cylinder injection valve for injecting fuelinto a cylinder of the engine, an intake passage injection valve forinjecting fuel into an intake passage connected to the cylinder, and anexhaust passage connected to the cylinder, the apparatus comprising: atemperature determining section that determines whether the temperatureof a component located in the exhaust passage is higher than or equal toa predetermined temperature; and an injection control section thatcontrols the in-cylinder injection valve and the intake passageinjection valve, wherein, when the temperature of the component ishigher than or equal to the predetermined temperature, the injectioncontrol section increases, without changing the total amount of fuelsupplied to the cylinder, the ratio of a fuel injection amount of theintake passage injection valve to a fuel injection amount of thein-cylinder injection valve compared to a case where the temperature ofthe component is not higher than or equal to the predeterminedtemperature.
 2. The apparatus according to claim 1, wherein theinjection control section sets the ratio of the fuel injection amount ofthe intake passage injection valve to the fuel injection amount of thein-cylinder injection valve according to an operational state of theengine.
 3. The apparatus according to claim 1, wherein, in a case wherethe temperature of the component becomes higher than or equal to thepredetermined temperature when the engine is operating in a fuelinjection mode in which fuel is injected only from the in-cylinderinjection valve, the injection control section causes the in-cylinderinjection valve to stop injecting fuel and causes the intake passageinjection valve to inject fuel.
 4. The apparatus according to claim 1,wherein the predetermined temperature is a reference value thatindicates that the component is in an overtemperature condition or atemperature lower than the reference value.
 5. The apparatus accordingto claim 1, wherein the predetermined temperature is a reference valuethat indicates that the component is in an overtemperature condition,wherein, when the temperature of the component is still higher than orequal to the reference value after the ratio of the fuel injectionamount of the intake passage injection valve to the fuel injectionamount of the in-cylinder injection valve is increased, the injectioncontrol section increases the total amount of fuel supplied to thecylinder.
 6. The apparatus according to claim 1, wherein thepredetermined temperature is a temperature lower than a reference valuethat indicates that the component is in an overtemperature condition,wherein, when the temperature of the component is higher than or equalto the reference value, the injection control section increases thetotal amount of fuel supplied to the cylinder.
 7. The apparatusaccording to claim 1, wherein the predetermined temperature is atemperature lower than a reference value that indicates that thecomponent is in an overtemperature condition, wherein, in a case wherethe temperature of the component becomes higher than or equal to thepredetermined temperature when the engine is operating in a fuelinjection mode in which fuel is injected only from the in-cylinderinjection valve, the injection control section causes the in-cylinderinjection valve to stop injecting fuel and causes the intake passageinjection valve to inject fuel, and thereafter, when the temperature ofthe component becomes higher than or equal to the reference value, theinjection control section also causes the in-cylinder injection valve toinject fuel, thereby increasing the total amount fuel supplied to thecylinder.
 8. The apparatus according to claim 1, further comprising anair-fuel ratio determining section, wherein, prior to the determinationof the temperature of the component by the temperature determiningsection, the air-fuel ratio determining section determines whether anactual air-fuel ratio of mixture of air and fuel is more than a targetair-fuel ratio, wherein, when the actual air-fuel ratio is more than thetarget air-fuel ratio, the injection control section increases the fuelinjection amount of at least one of the in-cylinder injection valve andthe intake passage injection valve such that the actual air-fuel ratioseeks the target air-fuel ratio.
 9. The apparatus according to claim 8,wherein, when the actual air-fuel ratio is more than the target air-fuelratio, the injection control section first increases the fuel injectionamount of the intake passage injection valve, and thereafter, when theactual air-fuel ratio is still more than the target air-fuel ratio, theinjection control section increases the fuel injection amount of thein-cylinder injection valve.
 10. The apparatus according to claim 1,wherein the temperature determining section determines whether thetemperature of the component is higher than or equal to thepredetermined temperature based on a parameter related to thetemperature of the component.
 11. The apparatus according to claim 10,wherein the temperature determining section determines the temperatureof the component based on the engine operational state and a fuelinjection amount ratio between the in-cylinder injection valve and theintake passage injection valve.
 12. The apparatus according to claim 1,wherein the component includes a catalyst that purifies exhaust gaspassing through the exhaust passage.
 13. The apparatus according toclaim 12, further comprising a temperature sensor that detects thetemperature of the catalyst.
 14. The apparatus according to claim 12,wherein the cylinder is one of a plurality of cylinders, and thecatalyst is one of a plurality of catalysts, wherein each catalystcorresponds to at least one of the cylinders, and wherein thetemperature determining section determines whether the temperature ofeach of the catalysts is higher or equal to the predeterminedtemperature, and wherein, in relation to the cylinder that correspondsto the catalyst the temperature of which is determined to be higher thanor equal to the predetermined temperature, the injection control sectionincreases the ratio of the fuel injection amount of the intake passageinjection valve to the fuel injection amount of the in-cylinderinjection valve.
 15. A fuel injection control method for an internalcombustion engine, wherein the engine has an in-cylinder injection valvefor injecting fuel into a cylinder of the engine, an intake passageinjection valve for injecting fuel into an intake passage connected tothe cylinder, and an exhaust passage connected to the cylinder, themethod comprising: determining whether the temperature of a componentlocated in the exhaust passage is higher than or equal to apredetermined temperature; and increasing, without changing the totalamount of fuel supplied to the cylinder, the ratio of a fuel injectionamount of the intake passage injection valve to a fuel injection amountof the in-cylinder injection valve compared to a case where thetemperature of the component is not higher than or equal to thepredetermined temperature, when the temperature of the component ishigher than or equal to the predetermined temperature.